Columbium addition agent



United States Patent Office 3,143,788 Patented Aug. 11, 1964 3,143,788 COLUMBIUM ADDlTION AGENT Thomas F. Kaveney, Cleveland, Ohio, and Rodneyd Merkert, Buffalo, N.Y., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Jan. 10, 1961, Ser. No. 31,633 1 Claim. (Cl. 29-182) The present invention relates to a novel process for producing columbium addition agents directly from oxidic columbium starting materials and high-carbon ferrochromium.

In the manufacture of stabilized stainless steels it is common practice to add to the steel bath a strong carbide-forming element such as columbium in an amount equivalent to about eight to ten times the carbon content. The columbium effectively combines with the carbon and prevents carbide precipitation during operations requiring heating of the steel, such as welding, thus avoiding the known deleterious eifects of carbon on the corrosion resistance properties of the steel.

Ferrocolumbium has been used for this application but because of its greater density the alloy generally sinks to the bottom of the steel bath and is slow to diffuse through the melt.

Ferrocolumbium is prepared by a rather complex and expensive process usually comprising fusion with iron or iron ore and a reductant to remove associated tin, followed by carbiding of the resulting slag to separate the columbium from the other constituents thereof and finally smelting the carbide with iron ore to produce ferrocolumbium.

It is an object of the present invention to provide a novel columbium-containing low-carbon ferrochromium addition alloy which is less dense and more readily soluble in molten metallic baths.

It is another object to provide a process for producing columbium-containing, low-carbon ferrochromium directly from oxidic columbium starting materials and highcarbon ferrochromium at low temperatures.

It is a further object to provide a process for producing substantially tin-free, columbium-containing low carbon, ferrochrornium directly from tin-containing oxidic columbium starting materials and high-carbon ferrochrornium at low temperatures.

The product satisfying the above mentioned objects comprises sintered, columbium-containing, low-carbon ferrochromium alloy addition agent having an apparent density ranging from 6.5 to 7.5 grams per cubic centimeter and being characterized by its ability to fioat on the molten melts to which they are added, and comprising 5 to 20 weight percent columbium, 45 to 65 Weight percent chromium, less than 2.0 percent silicon, less than 0.5 weight percent carbon, less than 0.10 Weight percent tin, less than 0.03 weight percent sulfur and the remainder iron and incidental impurities.

The process achieving the above-mentioned objects and providing the above-mentioned novel addition alloy comprises admixing particulate, high-carbon ferrochromium and particulate, oxidic columbium starting materials and a suitable binder, compacting the admixture to form a coherent body, heating the body in vacuo at a pressure less than about 3.0 mm. of Hg and to a temperature below about 1500 C. and reacting the carbon in the high-carbon ferrochromium with the oxygen in the oxidic columbium containing starting materials to cause reduction of columbium and evolution of carbon oxide, continuing heating until the evolution of carbon oxides substantiallly subsides and removing the resultant sintered, low-carbon, columbium containing, ferrochrornium.

It has been found, surprisingly and contrary to expectations, that, under the conditions specified in the process of the invention, columbium oxidic compounds are reduced to the metallic state by the carbon contained in high-carbon ferrochromium. The reason for this unexpected result is not known, but it is thought that interdifiusion of the ferrochromium and the columbium favorably alters the equilibrium of the reduction reaction. This is supported, in part, by data secured from X-ray diffraction examinations which disclosed the presence of the compound Fe Cb in the final product.

As disclosed in the US. Patent 3,091,624 issued on May 28, 1963, it is known that columbium metal may be produced by heating an intimate mixture of pure oxide and carbon. It has been found necessary to heat such mixtures to a temperature in the range of 2000 C. to 2200 C. in order to avoid the formation of columbium carbide as the final product rather than the desired pure metal. Columbium carbide has no use in stainless steel-making. Since columbium is utilized in stainless steel making to counteract carbon, there is no advantage to adding columbium carbide to stainless steel.

High-carbon ferrochromium commonly contains 4.5 to about 7.0 weight percent carbon. Low carbon ferrochromium commonly contains 0.025 to about 2.0 weight percent carbon. Throughout the remaining disclosure and appended claims, the above-identified high-carbon and low-carbon ferrochromium alloys will be utilized to indicate the reactants and products.

Columbium oxide starting materials are obtained from a variety of sources and the tramp oxides present in the starting material vary accordingly. Ta O Fe O Mn O SiO and tin oxides are commonly present in oxidic starting materials. Tin oxide is a common and persistent contaminant of columbium oxide starting material due to the close association of the oxides of tin and columbium in mother ores. Since tin is an undesirable element in steel, it must be eliminated from alloy addition agents used in steel-finishing. Virtually all oxidic columbium from known sources may be utilized as a starting material in the present process including tin-containing oxidic columbium starting materials.

Suitable binders include chromic acid, molasses, starches, chromium or iron formate solutions and the like.

The particle size of the material utilized in the starting admixture will affect the rate of the reaction. It has been found that the particle size of the oxidic columbium starting material should be in the range of from about 1-00XD to about 325XD (Tyler Mesh) and preferably about 10'0XD (Tyler Mesh). Particle size of high-carbon ferrochromium should be in the range of from about 200XD to about 40 OXD (Tyler Mesh) and preferably about 300XD (Tyler Mesh.)

The reaction in the present process is essentially a solid state reaction carried out in vacuo. The pressure during the reaction is maintained below about 3.0 mm. of Hg.

Heating rates are important. It is essential that the rate of temperature rise during the reaction does not cause the temperature of the pressed forms to exceed the eutectic melting point of the combination of compounds in the compacts. By the same token it is desired to cause sintering of the pressed forms. Incipient melting of the particle in the compact will seal the interstices of the compact and prevent outward flow of the carbon oxides resulting from the decarburizing reaction. The reaction will, therefore, be stopped. In many instances it has been found that sintering can be caused without incipient melting of the particle. This is due to mass diffusion of the metals between particles. In the present process the temperature is caused to increase at a rate whereby sintering does take place yet mass melting of the compact is not produced. Sintering will take place at temperatures well below the eutectic melting point of the product produced by the present process.

The maximum temperature utilized in the present process is 1500 C. Temperatures in excess of 1500 C. cause melting of the compacts. The lower temperature limit is about 1200 C. Temperatures lower than about 1175 C. cause slow reaction rates and poor heat penetration.

As previously stated the present process is essentially a solid state reaction. During the reaction, carbon from the high-carbon ferrochromium reacts with oxygen from the oxidic columbium starting material to cause evolution of carbon oxides. The temperature of the reactant mass is maintain-ed until the evolution of carbon oxides substantially subsides. At this point the reaction is substantially completed and the resultant product is cooled and removed from the furnace.

The molar ratio of oxygen in the oxidic columbium starting material to carbon in the high-carbon ferrochromium of the starting admixture is essentially maintained in the range of about 1.0 to about 1.2. The preferred molar ratio is about 1.10.

The product of the present invention is characterized by its density which ranges from 6.5 to 7.5 grams per cubic centimeter in addition to the above-noted compo sition.

Compaction of the start-ing admixture is accomplished in accordance with well known methods.

As stated in the objects, the present process can be advantageously utilized to produce columbium-containing low-carbon ferrochromium with a low tin analysis from oxidic columbium starting materials containing substan tial amounts of tin. This is accomplished by adding a sulfur-bearing material to the starting admixture in sufiicient amounts to react with the tin in the columbium starting material to form a tin sulfide. Tin sulfide is volatile under the operating conditions of the present process and is eliminated with the evolved carbon oxides.

It has also been observed that the addition of as little as 20 to 50 percent of the sulfur required to react stoichio metrically with the tin in the starting material results in satisfactory removal of tin during the process. This may be due to volatization of part of the tin as an oxide. At any rate the upper limit of sulfur utilized in the present process is about the stoichiometric amount required to react with all the tin in the starting materials.

The particle size of the sulfur bearing material in the starting admixture should range from 100 Mesh by Down to 325 Mesh by Down (Tyler Mesh) and preferably 100 Mesh by Down (Tyler Mesh).

The sulfur-bearing material utilized in the present process preferably has lower vapor pressure than tin sulfide for a given temperature. An example of a sulfur-bearing material which may be utilized in the present process is iron sulfide. It should be noted that any sulfur in the fer-rochrome may assist in tin removal. Elemental sulfur may possibly be utilized although it may volatize under the process conditions before reacting with the tin in the starting materials. This innovation permits an artisan to utilize columbium oxide starting materials containing tin to produce substantially tin-free, columbium-containing, ferrochromium, thus eliminating steps normally required to remove tin from columbium oxide to be utilized in the production of alloy addition agents.

The following examples will serve to illustrate the present process.

In all the examples the ferrochromium, the columbium source materials and the sulfur-bearing materials were nominally 300 (Tyler Mesh) by Down, (Tyler Mesh) by Down and 100 (Tyler Mesh) by Down respectively. The starch binder utilized in all examples was nominally 100 (Tyler Mesh) by Down in particle size. All the starting admixtures were pressed into coherent forms by standard techniques and a vacuum resistance furnace was utilized to heat the mixture.

EXAMPLE I An admixture was prepared by intimately blending; 11.8 pounds of oxidic columbium starting material containing 66.05 weight percent Cb O 6.5 weight percent Ta O 18.94 weight percent FeO, 2.18 weight percent MnO and 2.30 weight percent SnO 40.0 pounds of high carbon ferrochromium containing 66.68 Weight percent Cr, 24.08 weight percent Fe, 4.95 weight percent C and 0.061 weight percent sulfur; with 0.8 pounds of starch binder and 3.2 pounds of water. The admixture was pressed into bricks 5 inches by 5 inches by 10 inches and the bricks were dried at 400 F. for about 12 hours. The bricks were charged into a vacuum furnace, the furnace was sealed and the pressure was reduced to microns. The furnace temperature was raised to about 1000 C. over a period of six hours, held at 1000 C. for 4 hours and then raised to 1385 C. over a 5 hours period. A temperature of 1385 C. was maintained for 72 hours after which the furnace was allowed to cool at a rate dependent solely on the natural heat loss. The product obtained analyzed 55.0 weight percent chromium, 12.0 weight percent columbium, 0.49 weight percent carbon, 1.97 weight percent silicon, 0.18 weight percent tin, 3.2 weight percent 0 and 0.018 weight percent sulfur, 0.01 weight percent CaO-l-MgO, 0.82 weight percent A1 0 and the remainder iron.

EXAMPLE II Following the procedure of Example I two mixtures were prepared analyzing as follows:

Mixture I: 11.7 pounds of oxidic columbium starting material containing 95.5 weight percent Cb O 1.3 weight percent Ta O and 0.034 weight percent FeO; 40 pounds of high-carbon ferrochromium containing the same as that utilized in Example I: 1 pound of starch binder and 3.2 pounds of water. The molar ratio of oxygen to carbon was 1.2.

Mixture II: 13.7 pounds of oxidic columbium starting material containing 51.24 weight percent Cb' O 0.5 weight percent Ta O 19.98 percent FeO, 8.61 weight percent SiO and 0.02 weight percent SnO 40 pounds of high-carbon ferrochromium having the same analysis as Example I; 1 pound of starch binder and 3.2 pound's'of water. The molar ratio of oxygen to carbon was 1.2.

The above mixtures I and II were pressed into bricks of the size shown in Example I and dried and furnac'ed in the manner shown in Example I except a part of the bricks resulting from each mixture I and II above were treated for 2 hours at 1360 C. following by 20 hours at 1450 C. The products resulting from mixture I above analyzed 56.33 weight percent Cr, 17.4 weight percent Cb, 0.01 weight percent C and 1.6 weight percent 0 and the remainder iron and incidental impurities.

The products resulting from mixture II analyzed 57.10 weight percent Cr, 9.0 weight percent Cb, 0.32 weight percent C, 2.3 weight percent and the remainder iron and incidental impurities.

The remaining bricks resulting from each of the above mixtures I and II were treated at a temperature of 1360" C., for a period of 40 hours. The bricks resulting from mixture 1 contained 0.06 weight percent C and the bricks resulting from mixture H contained 0.10 weight percent carbon.

EXAMPLE III Three mixtures were prepared to illustrate the effect of the molar ratio of oxygen to carbon on carbon removal and also elimination of tin by sulfur addition.

The columbium source material contained the follow- Compound: Weight percent Cb O 66.51 Ta O 6.25 Fe() 18.43 SiO 2.06 SnO 2.96

The high-carbon ferrochromium contained the followmg:

Element: Weight percent Cr 67.02 Fe 24.20 C 4.90 S 0.06

Iron sulfide having 36 weight percent sulfur was utilized as the sulfur-bearing material.

The proportions in pounds in each mixture as follows:

Material Mixture Mixture Mixture I II III Cb source 60 10. 9 11.4 High-carbon ferrochrornium 200 40. 0 40. 0 Starch binder e. 5 1.0 1.0 FeS none 0. 14 0.14 Vater 0 3. 2 3. 6 0 /0 molar ratio 1.1 1. 0 1.05

The mixtures were pressed each into pillow-shaped pellets approximately 2 inches by '2 inches by 1.5 inches. The same heating procedure was employed as shown in Example I. A temperature of 1360 C. was maintained for 36 hours in treating the pellets from each mixture. The products resulting from each mixture contained the following:

Mixture Percent Percent Percent Percent Percent Percent Fe Cr Cb C S S11 TableI SOLUTION DATA FOR COLUMBIUM-BEARING ALLOY iEIgQlggIIONS MADE TO TYPE 430 STAINLESS STEEL AT Per- Per- Percent Solution Alloy Composition cent cent Recov- Time,

y Cb Cb in ery Sec.

Added Steel Present Alloy 0.47 94 28 Present Alloy. 0.34 62 24 Std. FeCb 0.42 84 20 Std. Fe'IaCb 0.39 88 60 Table II MEI/PING RANGE DATA Temperature, 0. Alloy Composition Solidus Liqnidns C! 56.66.-- Present Alloy g '?f 1, 545-1, 570 1, 530-1, 585 C 0.01

Std. FeOb 1,680-1, 685 1, 720%, 730

Cb 15.61.". Std. FeTaCb lf fi- 1, 700-1, 1, 780-1, 785 C 0.20

Table III APPARENT DENSITY Present Alloy 6.9-7.2 g./cm. 0.25/ftfi. Std. FeCb--..- 8.4 0.30/it}. Std. FeTaCb 0.30 Type 430 Steel at 1,530 0. (Approx.)..

It should be noted that all the above alloys of the present invention floated on molten type 430 steel at a temperature of about 1530 C.

What we claim is:

A product of manufacture comprising a sintered, columbium-containing, low-carbon ferrochrornium alloy addition agent having an apparent density ranging from 6.5 to 7.5 grams per cubic centimeterand being characterized by its ability to float on molten steel melts and consisting essentially of 5 to 20 weight percent columbium, 45 to 65 Weight percent chromium, less than 2.0 Weight percent silicon, less than 0.5 weight percent carbon, less than 0.10 weight percent tin, less than 0.03 weight percent sulfur and the remainder iron and incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS 1,814,719 Marden et al July 14 ,1931 2,203,214 Doom June 4, 1940 2,333,573 Kalischer Nov. 2, 1943 2,763,918 Megill Sept. 25, 1956 2,818,339 Dodds a Dec. 31, 1957 2,839,379 Erasmus June 17, 1958 2,849,789 Thomson Sept. 2, 1958 

